The overall objective of this project is to assess a potentially valuable therapeutic approach in Multiple Sclerosis (MS), a chronic, progressive or relapsing disorder in which mor effective treatments are greatly needed. The hypothesis to be tested is that inhibiting the biologic effects of the cytokine Interleukin-12 (IL-12) by treatment with a recently developed humanized anti-IL-12 monoclonal antibody will interfere with the pathogenesis of MS. As a result, there would be a reduction in the relapse rate and progression of this disease. Specific Aims: a. Phase I trial of A/IL-12 Proposed is a one-year pilot trial )Phase I) of monthly infusions of a humanized anti-IL-12 monoclonal antibody (hereafter called A/IL-12) in MS patients with histories of recent frequent (greater than or equal to 2 per year) clinical exacerbations. Specific aims include: i) Assess safety and tolerability hematologic, hepatic, dermatologic, renal, pulmonary, etc. ii) Assess effects on host defenses against infection cell mediated and humoral immunity. iii) Determine whether there is a beneficial effect on A/IL-12 on the frequency and severity of MS clinical relapses and disability progression. iv) Determine if A/IL-12 treated MS patients have an unexpected worsening of their clinical manifestations. b. Phase II- Trial of A-IL-12 for assessment of efficacy of A/IL-12 in reducing the relapses and progression of MS.